1. Field of the Invention
The present invention generally relates to abrasive cutters useful in creating subterranean boreholes. More specifically, the present invention is directed to a compact cutter having superior impact resistance by having reduced residual stress.
2. Description of the Prior Art
Polycrystalline diamond compacts (PDC) are commonly used in oil field drilling and machine tools. A PDC is a synthetic form of diamond that is made by pressing diamond powder and cobalt onto a cemented tungsten carbide substrate. In the press, the cobalt becomes liquid and acts as a catalyst for diamond grain growth. The result is a highly abrasive, e.g. roughly 90% as abrasive as natural diamond, and environmentally resistant component which is very adaptable to drilling systems for resistant rock formations.
Although PDC is resistant to abrasion and erosion, a PDC compact cutter demonstrates several disadvantages. The main components of the PDC system, diamond and tungsten carbide, are brittle materials subject to impact fracturing. Moreover, because tungsten carbide and diamond have different coefficients of thermal expansion, there are residual stresses in a PDC system because the tungsten carbide demonstrates greater contraction during the cooling phase than that of the synthetic diamond.
As a result of the afore referenced disadvantages, attempts have been made in the art to limit the affects by modifying the geometry at the interface between the diamond and the tungsten carbide. Such modifications have usually taken the place of an irregular, non planar interface geometry. The most beneficial resultant of the non-planar interface, defined as any design where the interface between the diamond and tungsten carbide is not a circular plane, is the redistribution of residual stresses. Redistributing residual stresses allow the PDC manufacturer to increase the diamond thickness, thereby providing increased wear resistance. An irregular interface is advantageous since brittle materials are more resistant to compressive loads than tensile loads. The existence of a flat interface causes tensile stress plane between the diamond and tungsten carbide. This plane generally defines a main failure locus for delamination of the diamond layer.
One cutter which first utilized a non-planar interface geometry was the "Claw" cutter, so named as a result of the wear pattern of a worn cutter which looked like the remnants of claw marks. The interface of the "Claw" cutter, when viewed in cross section, consists of a plurality of parallel ridges and grooves disposed across the diameter. The "Claw" cutter provided advantages in the areas of wear resistance, but demonstrated a number of disadvantages which included the need to orient the cutter in order to position the parallel diamond inserts normal to the cutting surface. This required orientation of the cutter vis-a-vis the drill bit body complicates the manufacture process.
The so called "Ring Claw" cutter adopted a similar design to that of the Claw cutter except that the Ring Claw included a enhanced thickness ring of synthetic diamond which bounded a series of parallel inserts which also includes diamond of an enhanced thickness. The Ring Claw cutter demonstrated improved wear resistance over the Claw cutter, but when the outer diamond ring became worn, demonstrated similar disadvantages as to the need for precise orientation vis-a-vis the work surface.
Another prior art cutter is colloquially known as the "target cutter", and is characterized by an alternating grooves and ridges formed on the cutting face in the form of a target. The target cutter, while addressing the issue of orientation presented by the "Ring Claw cutter," demonstrated vulnerability to hoop stresses. Hoop stresses are created on the bounding ridges of tungsten carbide positioned interior to grooves filled with synthetic diamond. Hoop stresses are caused by uninterrupted concentric grooves and ridges in the PDC. During cooling of the PDC after pressing, the tungsten carbide ridges will contract and compress on the synthetic diamond rings disposed in the internal grooves. Such contraction simultaneously pulls the tungsten carbide substrate away from diamond disposed in external rings. These differential stresses create a tensile load between all of the internal tungsten carbide ridges and synthetic diamond disposed in all external grooves. such stresses can be severe enough to completely delaminate the synthetic diamond layer. A more common failure is the creation of stress zone in the interface, where friction due to impact can originate.